Known as one example of the nanometer standard prototype is a standard sample for calibrating the accuracy of an atomic force microscope (AFM), a scanning tunneling microscope (STM), and the like. The AFM and the STM are used for, for example, measuring a structure of the order of nanometers. Therefore, a standard sample used for the calibration needs to have a standard length (such as the height of a step) with an extremely high accuracy. Particularly in recent years, it is demanded that a semiconductor crystal surface, or the like, be accurately observed and measured in a minute area of the order of nanometers. Thus, a high accuracy of calibration of the order of Angstroms or less is required when the AFM or the STM is calibrated by using a standard sample. Non-Patent Document 1 and Patent Documents 1 to 3 disclose this type of standard sample or a method for manufacturing the standard sample.
A standard sample disclosed in the Non-Patent Document 1 is a silicon step substrate having a single-step structure on a main surface thereof. It is manufactured such that the step has a height (0.31 nm) corresponding to two silicon atomic layers. The silicon step substrate having such a single-step structure on the main surface thereof is manufactured from a single crystal silicon substrate through the following method.
Firstly, a single crystal silicon substrate whose main surface includes a surface that is slightly inclined from a (111) plane toward the [11-2] direction is cut out into an appropriate size. Then, this substrate is subjected to a proper treatment, and then put into a vacuum chamber in which degassing is performed. After the degassing is performed sufficiently so that an ultrahigh vacuum atmosphere (about less than 6.5×10−7 Pa) is produced, heating up to 1100 to 1200° C. is performed and this state is maintained for about 10 minutes. Then, rapid cooling down to room temperature is performed, and the vacuum chamber is purged with dry nitrogen. Under such a condition, the substrate is taken out. In this manner, a silicon step substrate having a single-step structure on a main surface thereof can be manufactured.
A standard sample disclosed in the Patent Document 1 is manufactured from a silicon wafer through the following method. The silicon wafer is smoothed sufficiently, and then a thermal oxide layer is grown. A pattern mask is made through a photolithography process, and then an etching process is performed. At this time, using an etching agent having an extremely low etching rate improves the accuracy of estimate of the etching rate, which enables the amount of etching to be set to a predetermined value. The pattern mask is removed, and thereby a pattern step with a high accuracy can be formed.
A standard sample disclosed in the Patent Document 2 is manufactured from a single crystal sapphire substrate including, in a main surface thereof, a surface with a (0001) plane or with a plane having an off angle of 10 degrees or less from a (0001) plane, through the following method. Appropriate polishing is performed on the single crystal sapphire substrate, and then a plurality of recesses are formed in the main surface of the substrate. The single crystal sapphire substrate having the recesses formed thereon is subjected to a heat treatment in the atmosphere. As a result, a step-terrace structure concentrically centered to the bottom of the recess is obtained. The height of one of the step is 0.22 nm. In the above-described manner, a sapphire substrate having steps with a high accuracy can be manufactured.
A method for manufacturing a standard sample disclosed in the Patent Document 3 is a method that enables the recesses, which are formed through the method disclosed in the Patent Document 2, to be formed accurately. In this method, an indenter made of a material harder than sapphire is pressed against a sapphire substrate, so that recesses are formed. The size and depth of the recess is adjustable in accordance with a load used to press the indenter.